Diarrhea/loose stool every couple of weeks

I have two cats (brothers), 1y and 4 months old. Domestic long hair. They grew up on Royal Canin gastrointestinal roughly for the first year of their life because Woody (the ginger one) kept having loose stool, which was normalised with the gastro food. At 1 year old, our vet told us to try adult food (not gastro) to see how they would behave, and they were fine for about 2 months. We were giving them Seriously Good chicken and fish, which is all natural. Since the beginning of September, Slinky (the black one) has diarrhea/loose stool every couple of weeks, which resolves after a couple of days by going back to gastrointestinal + probiotics. Just to come back again after the time specified above. They’re indoor cats, vet says it’s unlikely they would have parasites and I couldn’t see any signs pointing towards those. The cats are otherwise healthy, not lethargic, playing and eating as usual. What can I do about it? Should I go back fully to gastrointestinal? Although they don’t seem to like the adult Royal Canin gastro wet food at all. Any other possible causes/solution? Thank you!

Hello!  The first thing to say is that there are 'gastrointestinal foods' and 'gastrointestinal foods.' All food goes through the gasterointestinal tract; the exact definition of 'gastrointestinal' is therefore a bit shady and varies from manufacturer to manufacturer.  Therefore, my saying 'yes - you should give your cat other gastrointestinal food' may be meaningless.  If your vet deems that a particular food is nutritionally appropriate to your cat in the way that you are using it, AND the symptoms appear to clear up exclusively on it, then it sounds as though it may be.  If, however, you are concerned about this food then it makes sense to tell the vet that; there are multiple pet foods around and they are likely to have a second suggestion. It is not uncommon for cats to 'turn their noses up' at veterinary cat foods in hope of something with a stronger taste.  Historically, many cat-foods had additives eg salt or high fat, to make them extra-tastey.  Without that, cats have no excessive drive to eat once they are full.  If you measure how much your cat is eating per day eg in grammes, then your vet who knows the weight of your cat, will usually let you know whether the quantity being consumed is appropriate. I hope that something there helps.